Head-up displays (HUDs) provide pilots with two superimposed images. One image can be a view of the outside world while the other image can be symbology relating to that view of the outside world.
The ability to display concurrent images is provided by an optical element in the HUD called a combiner. The combiner is typically a partially silvered mirror which both transmits the view from the outside world to the pilot and reflects the second image toward the observer from another direction. This second image can also be collimated by the combiner so that it is focused at infinity, obviating the need for the pilot's eyes to refocus when he diverts his attention from one image to the other. Combiners also provide an HUD with optical power.
Using a holographic optical element (HOE) as a HUD combiner makes the second image brighter and larger. However, present HOEs display only a single color; the design of multicolor HOEs is even more complex than the design of single color HOEs. Also, additional equipment and/or greater volume will be required by multicolored HUDs in the already crowded and complex environment of an aircraft cockpit.
HOEs are typically recorded using lasers for the coherent sources of light, while HUDs typically generate their images from a source such as a cathode ray tube (CRT). The phosphors used in multicolor CRTs do not create light in the same wavelengths as those available from the lasers used in the recording process. This mismatch, even if only slight, has a very detrimental effect on the diffraction efficiency of the reconstructed images produced by HOEs.
Several investigators have dealt with difficulties providing bright images, and/or multicolored displays by using HOEs. None, however, has provided a satisfactory concurrent solution to these problems suitable for use in a HUD.
In their paper "Wavefront Reconstruction with Diffused Illumination and 3-Dimensional Objects," Journal of Optical Society of America, Volume 54, No. 11, November 1964, Leith and Upatnieks discussed the possibility of reproducing a wavefront containing a multicolor, 3-dimensional image. The system they describe presumes the illumination of the original object by separate coherent beams, one for each of the three primary colors. Each of the primary colors illuminates the object by means of a separately placed mirror. These record beams then form interference patterns with a coherent beam of the same color to create the hologram for that particular primary color.
To reconstruct this holographic image the resulting hologram must be illuminated by three readout beams, having the proper wavelengths and illuminating the hologram from the proper directions. The wavelengths used in the three readout beams must be identical to the respective wavelengths in the three record beams. The apparatus required is cumbersome and will not comply with the strict volume requirements in most HUD applications.
In their paper, "Multicolor Wavefront Reconstruction," Applied Physics Letters, Volume 7, No. 3, Aug. 1, 1965, Pennington and Lin describe a method for reconstructing multicolor wavefronts where the recording geometry for each of the colors satisfies the Bragg condition. The reconstructed wavefronts are thus very bright. Again, however, the method requires that the hologram be recorded at the same wavelengths as the intended reproduced wavelengths. This will not be the case where holograms are recorded through use of lasers but the images are reproduced by means of CRTs.
U.S. Pat. No. 3,532,406 discloses a method for reconstructing multicolor images from reflection holograms in which both the reference and object beams contain two or more colors and each projects from a different direction onto the photographic plate. This method suffers from the same deficiencies as the approach taken by Leith and Upatnieks. U.S. Pat. No. 4,415,225 and U.S. Pat. No. 3,737,212 suffer from similar deficiencies for HUD applications.
Consequently, a need exists for a method of producing multicolor HOEs which produce bright images in the direction of the pilot and which are illuminated from a single direction by images produced by a CRT, thereby providing a compact HUD.